Add VPI C example

Author: kaushalmodi

tests/basic/README.org

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+#+title: Nim/SystemVerilog VPI Example
+
+This directory contains the "Hello" example from [[https://www.asic-world.com/verilog/pli6.html][this asic-world PLI
+tutorial]] translated from C to Nim.
+
+- To run the Nim code with SystemVerilog, run ~make~ in this
+  directory.
+- To run the original C code from the tutorial, cd to the ~orig/~
+  directory in here, and run ~make~ there.

tests/basic/orig/libvpi.c

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+// https://www.asic-world.com/verilog/pli6.html
+
+#include "vpi_user.h"
+
+void hello() {
+  vpi_printf("\n\nHello!!\n\n");
+}
+
+// Associate C Function with a New System Task
+void registerHelloSystfs() {
+    s_vpi_systf_data task_data_s;
+    p_vpi_systf_data task_data_p = &task_data_s;
+    task_data_p->type = vpiSysTask;
+    task_data_p->tfname = "$hello";
+    task_data_p->calltf = hello;
+    task_data_p->compiletf = 0;
+
+    vpi_register_systf(task_data_p);
+}
+
+// Register the new system task here
+// https://cdecl.org/?q=void+%28*vlog_startup_routines%5B+%5D+%29+%28%29
+void (*vlog_startup_routines[ ] ) () = {
+    registerHelloSystfs,
+    0  // last entry must be 0
+};

tests/basic/orig/tb.sv

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+../tb.sv